The present invention relates generally to a rolling loop type of large screen motion picture projector such as has been described in U.S. Pat. Nos. 4,114,996 and 4,365,877 and in particular to a film support surface for such a projecter and to the cleaning of the support surface. There are two principal objects of this invention; first to assure that each frame of the film is properly positioned, in a stable and secure manner, relative to the projection lens focal plane, and second, to assure that the film and lens are free from particles of dust, dirt, hair and other contaminants which would be distracting to the audience, especially when seen on the screen magnified several hundred times.
A rolling loop type of large screen motion picture projector is conventional, e.g., see U.S. Pat. Nos. 4,114,996 and 4,365,877. Film enters such a projector through a film inlet guide where a portion of the film which is exactly one film frame in length is formed into a loop. Between the point on the inlet guide where the loop is formed and the aperture where the film frame is projected onto the screen is a distance of approximately five film frames in the film inlet guide. Hence, at any time during projection, the film inlet guide contains five stationary frames of film, registered to the aperture, and one frame of film in the loop which is moving in a circular path toward the aperture. The movement of the loop over the film in this path is very similar to that of a single wave moving over the surface of water. As the loop moves along the film path, the film frame which is in the loop replaces the film frame in the stationary film path that is just passed over. As the loop passes over the aperture, the next frame to appear in the aperture (on the screen) is the frame that had just previously been the one which was in the loop. Thus, the picture changed from one frame to the next one in proper sequence without moving the film or sliding on the inlet guide.
For structural reasons, as well as other reasons to be explained later, the film inlet guide is curved. Therefore, the loop and the film must follow a similarly curved path to enter and to exit the aperture. To accommodate this requirement, a circular motion is imparted to the loop by a component called a rotor which rotates through a radius of curvature and around an axis of rotation which coincides exactly with the axis and radius of curvature of the film guide. Furthermore, the film frame in the aperture must follow the same curvature and center of curvature that it followed when it was in the guide. For this reason, the rearmost element of the projection lens is made to the same radius and center of curvature as the film inlet guide. The surface just designated is caused to become the primary focal plane by designing the projection lens so that its focal plane is this surface. The film then is held in optical contact with this surface, thereby assuring that the film is held securely in the proper plane and that its geometric shape is properly curved. This element of the projection lens is also proportioned appropriately so that it provides the horizontal boundaries on both sides of the framing aperture. This element constitutes the focal plane and aperture of the entire optical system and although being transparent is very sharply focused on the screen. Any dust, dirt, hair or other contaminating materials present thereon would appear on the screen magnified several hundred times. The large nondescript black blobs thus produced would be very unpleasant and distracting to the viewing audience.
As the film moves into and through the projector, particles of contamination such as dust, dirt, hair, etc., carried thereby will be deposited on the lens/film support surface. The film has been previously cleaned by other means before entering the film guide, but after the loop is formed it picks up additional contamination. This is because the projector requires the use of a large film format containing very large frames (individual picture areas) several times larger than those in more common usage. When this film is projected at the standard rate of 24 frames per second, it is moving through the air at a much higher speed that the more common, smaller films at the same frame rate and it therefore builds up a significantly higher intensity of electrostatic charge. For this reason, particles of dust, dirt, hair, etc., are attracted to, and adhere to the film. Although, as mentioned before, the particles are removed by other means before the film enters the inlet guide, additional contamination can occur after the loop is formed but before arriving at the aperture. Then when a film frame is in the aperture, as previously stated, it is stationary and it is in optical contact with its support surface which is the rear surface of the rear element of the projection lens. During the short period of time that each frame is in this position of optical contact with the lens/support surface, it can deposit thereon a small amount of contamination, so that after a period of about 15 minutes, the contamination becomes excessive and very noticeable on the screen and must be removed. The running time of most motion pictures is substantially greater than 15 minutes and it is therefore necessary to clean the lens surface at regular intervals such as may be required during the presentation of a film without stopping the show. One object of the present invention is to provide such a cleaning action which is effective and longer lasting.
The rear surface of the rearmost element of the projection lens is actually one of a group of four surfaces (a group of two lens elements) which together have the main function of supporting the film in a manner optically modifying the curved shape of the film in the aperture so that it appears flat on the flat screen.
It is common, in optical parlance, to euphemistically refer to this function as a field flattener: and hereafter, this group of two lens elements together will be referred to as a field flattener even though it performs other important functions not implied by that title. As stated previously, there are two lens elements in the field flattener group, but there are also two field flattener groups and each of these groups can function interchangeably as rear elements of the common projection lens so that one field flattener can be substituted for the other while a show is being presented and the difference would not be discernible.
The field flattener elements are actually cylindrical lenses having the general shape of a cylindrical segment standing on end with its cylindrical axis in a vertical direction. In lenses of this type, the optical power is entirely along the horizontal axis; that is to say, there is no power at all in the vertical axis. Hence, the elements can be moved in a direction parallel to the vertical cylindrical axis, without producing a discernible shift or displacement of the projected image. The field flatteners are therefore mounted in a carrier module, which is capable of moving up and down in a vertical direction, and they are arranged in such a way that when one is centered in the aperture and aligned with the projection lens and supporting the film frame which is in the aperture, the other is above or below the aperture and is therefore not visible on the screen.
During a projecting operation, there may tend to occur a condition commonly referred to as "jump" wherein the film in the aperture undergoes a slight vertical movement. When this occurs, a corresponding, but greatly magnified, vertical movement of the projected image appears on the screen, which is unpleasant for the audience. It is desirable to minimize that condition.